: : Functionally, you can make the change is size, but if it is going to be inspected your code may require a continuous 4" pipe rather than a total of 4" venting. You can offset the vent pipe with as many elbows as necessary.

: : : I have a 4inch cast iron vent stack that is in a wall in my kitchen (only thing that vents to this is one toilet in the basement). I want to move this wall in my kitchen to make it bigger. Is it feasable to go from 4inch cast iron to 3inch plastic? Also is there a limit to how many 90 degree turns I can make on this vent stack, I will have to make a total of three for this project. Thank you for your help.

: A downside of plastic is the noise, I completely removed my cast iron and now regret saving at least a portion due to noise...

Plastic drain piping certainly conducts more noise that cast iron. I would think plastic vent pipe would not but it might echo. The size of the vent depends on the total vents for the house. They must equal the cross circumference area of the house drain which is usually four inch.LonnythePlumber